My son's idea of a toboggan is a six-foot sheet of heavy blue plastic that's rolled up somewhere in the basement. So, at the Cabin John toboggan slide, he was surprised to see the real thing - a padded wooden sled that seats four and curls up at only one end.

We spent an afternoon there one recent weekend learning the ups and downs of tobogganing.

Even a sophisticated 11-year-old city kid like my son was amazed to be sledding in 60-degree November weather. The park's two icy slides are perpetually frozen by tubes of freon underneath.

The tobogganining started off well, as things will when you're unself-conscious - you don't know you don't know what you're doing. Here, the natural way was the right way: We leaned forward on the toboggan in a kind of anticipation, and took off.

While fellow sledders said the greater the weight the faster you go, our daring duo out-slid sleds of four or five teenagers, and we felt great.

But somehow the doldrums came and we lost our rhythms when we began to think. Why did we defy the weight maxim? We experimented. We saw the last person on another toboggan lying face up, feet in the lap of the guy in front of him. Protesting and trying to recall high school physics, I did same at my son's suggestion. We didn't even make it to the bottom.

We trudged back uphill. The sled seemed heavier to carry. On the ride down, again we lost to racers on the next sled - three other 11-year-olds, a bitter defeat.

We walked only part way back uphill, parking our toboggan in a rack at the warming lodge.Inside by th fireplace, we drank Cokes and talked of all the peoplewe could have invited to come - to add weight and carry the toboggan while we took a break.

Back on the hilltop platform, we were determined to make a long slide. We flew off, only to lose speed on the slope and again suffer the embarrassment of riding a sled that poops out before the end of the run.

On theway back up, we compained that we were two and three, wins and losses. A sled of five people streaked by. My son fumed and dropped his end of the toboggan.

He bought a bag of M&Ms at the snack bar and gave me two.

From the platform, we watched the others. We asked the attendant who releases the sleds onto the slope, "Does it make you go faster to lie down?" "No, I don't know why they do that." "Does it make you go faster to rock back and forth?" "No, but it makes you feel like you're moving faster."

Then, the attendant said, "The trick is to put all your weight forward." Vindication.

We scrambled on to our toboggan and curled forward. I'll never be sure if the attendant didn't give us a little push, because we went catapulting off the tray that launches the sleds onto the ice. We slipped and bounced along so fast I kept hitting my head on my son's back. When we got to the bottom, past the ice chute's end, it took 15 feet of pavement to stop us.

My son was instantly energized. "Everyone was watching us! Did you see that man's head move? It went . . ." To show me how, he turned his head in a quick sweep.

I hadn't noticed. My eyest were closed the whole way down.